In recent years and continuing, organic thin-film transistors using organic semiconductor materials are under intense study. The advantages of using organic semiconductor materials in transistors are flexibility, larger areas, simplification of a process due to a simple layer structure, and an inexpensive manufacturing device.
Furthermore, a printing method is employed so that manufacturing costs are significantly reduced compared to conventional Si-based semiconductor devices. Moreover, thin films and circuits can be formed simply and conveniently by employing the printing method, a spin coating method, and a dipping method.
One of the parameters indicating properties of such an organic thin-film transistor is the Ion/Ioff ratio of electric current. In an organic thin-film transistor, the electric current (Ids) flowing between source/drain electrodes in the saturation region can be expressed by the following formula (1),
                              I          ds                =                              μ            ⁢                                                  ⁢                          C              in                        ⁢                                          W                ⁡                                  (                                                            V                      G                                        -                                          V                      TH                                                        )                                            2                                            2            ⁢                                                  ⁢            L                                              (        1        )            where the field-effect mobility is (μ), the capacitance per unit area of a gate insulating film is Cin=εε0/d, where ε is the relative dielectric constant of the gate insulating film, ε0 is the dielectric constant of a vacuum, and d is the thickness of the gate insulating film, the channel width is (W), the channel length is (L), the gate voltage is (VG), and the threshold voltage is (VTH).
This formula indicates that, in order to increase the on current, it is effective to (1) increase the mobility, (2) decrease the channel length even more, and (3) increase the channel width. Furthermore, the field-effect mobility is largely dependent on material properties, and therefore, materials for increasing the mobility are being developed.
Meanwhile, the channel length results from the element construction, and therefore, the element construction has been devised in an attempt to increase the on current.
Generally, the channel length is reduced by reducing the distance between source/drain electrodes (electrode interval).
Organic semiconductor materials originally do not have high mobility, and therefore, the channel length is required to be no more than 10 μm, more preferably 5 μm or less.
One method of accurately setting a short distance between the source/drain electrodes is photolithography, which is employed in an Si process, including the following steps. (1) Apply a photoresist layer on a substrate with a thin-film layer (resist application). (2) Remove the solvent by heating (prebaking). (3) Irradiate ultraviolet rays through a hard mask having a pattern rendered thereon with a laser beam or an electron beam based on pattern data (exposure). (4) Remove the exposed resist with an alkaline solution (developing). (5) Harden the resist of the unexposed part (referred to as pattern part) by heating (postbaking). (6) Dip into etching liquid or expose to etching gas to remove the thin-film layer of portions without resist (etching). (7) Remove the resist with an alkaline solution or an oxygen radical (resist separation). The aforementioned steps are repeated each time after a thin-film layer is formed, to thereby complete an active component. However, the overall costs are increased due to expensive facilities and a time-consuming process.
Meanwhile, other attempts are being made to form electrode patterns by a printing method using an inkjet apparatus in order to reduce the cost.
In inkjet printing, the electrode pattern can be directly rendered, and therefore, the material utilization rate is high. Thus, the manufacturing process may be simplified and costs may be reduced. However, the jetting precision of inkjet printing is limited due to the difficulty in reducing the amount of jetted ink and machine errors. Thus, it is difficult to form patterns of 30 μm or less, and it is impossible to make the electrode interval as short as 5 μm. This means that it is difficult to manufacture a high-precision device with an inkjet apparatus alone. Accordingly, some device is necessary to attain high precision. One approach is to perform work on the surface onto which ink is jetted.
For example, there is a method of using a gate insulating film made of a material whose critical surface tension (also referred to as surface free energy) changes by receiving energy such as ultraviolet rays (see Patent Document 1). Ultraviolet rays are irradiated through a mask only onto the portions where the electrodes are supposed to be fabricated, to create high surface free energy portions on the surface of the insulating film. An electrode material including water-soluble ink is inkjetted onto these portions, so that electrodes are fabricated only on the high energy portions. Accordingly, high-precision electrode patterns can be formed on the gate insulating film. By employing this method, even if ink droplets are jetted onto a borderline between the high surface free energy portion and a low surface free energy portion, the droplets can move over to the high energy side due to the difference in energy. As a result, it is possible to create patterns with uniform lines. This method is advantageous in that an electrode interval of 5 μm or less can be realized. However, ultraviolet rays, more specifically, ultraviolet-C rays having a short wavelength of 300 nm or less are irradiated onto the gate insulating film, and therefore, the insulating film is affected and the insulating properties become degraded.
In another example, the gate insulating film is laminated with a film made of a material whose surface free energy changes by receiving ultraviolet rays (see Non-patent Literature 1). By the same method as that of Patent Document 1, portions with different levels of surface free energy are created on the film by irradiating ultraviolet rays, and electrode patterns are created by an inkjet method. The advantage of this technique is that functions are separated into the layer in which the insulating properties are retained and the layer in which the surface free energy changes. However, because ultraviolet rays are irradiated on the gate insulating film, there still remains the problem that the insulating film is affected and the insulating properties are degraded. As a result, gate leakage is increased and it is only possible to produce a device having a small Ion/Ioff ratio.
Non-patent Literature 1 reports an attempt of mitigating this problem by increasing the thickness of the gate insulating film (approximately 1 μm) in order to reduce the amount of ultraviolet rays being transmitted to the substrate layer. However, as indicated by formula 1, if the thickness of the gate insulating film is increased, the extracted current value Ids is decreased. As a result, it is only possible to produce a device-having a small Ion/Ioff ratio.
Consequently, it is necessary to increase the applied voltage VG in order to increase the Ion/Ioff ratio. As a result, it is difficult to produce a low power consuming device.
Patent Document 1: Japanese Laid-Open Patent Application No. 2005-310962
Non-patent Literature 1: The Japan Society of Applied Physics, The 52nd Spring Meeting, 2005, Meeting proceedings, p. 1510
As described above, by the method of fabricating portions of high surface free energy and portions of low surface free energy on a gate insulating film with ultraviolet rays or electron beams, it is possible to fabricate high-precision and high-density electrode patterns that are difficult to fabricate by the conventional printing method. However, a problem arises in that the insulating properties of the gate insulating film become degraded by receiving high energy light beams. Therefore, it is necessary to mitigate the adverse effects caused by irradiating high energy light beams.
Accordingly, there is a need for a laminated structure, an electronic element using the same, a manufacturing method therefor, an electronic element array, and a display unit, in which insulating properties of a gate insulating film are not degraded even if high energy light beams are irradiated on the gate insulating film.